Goals for the current year include: (1) establishing whether the decreased mitogen responsiveness observed in tumor-bearing animals is due to the presence of a suppressor cell; (2) identifying the nature of this suppressor cell; and (3) developing of techniques to assess tumor immunity in tumor-bearing animals. Newborn Fischer 344 rats were inoculated intracranially with avian sarcoma virus (ASV) and the mitogenic responsiveness of spleen and thymus cells determined. The results showed that spleen cells from tumor-bearing animals were significantly less responsive (40% to 70%) to various concentrations (0.1 to 50 microgram) of concanavalin A (Con A) as compared with the splenic response of age-matched normal animals. The response of thymocytes from tumor-bearing animals to Con A was similar to that of normal animals. Con A responsiveness of spleen cells from tumor-bearing animals returned to normal control levels by removal of a glass bead adherent cell population. Addition of this adherent cell population to glass bead nonadherent cells resulted in suppression of a subsequent Con A response. The adherent cell population was characterized to be resistant to treatment by rat antithymocyte serum in the presence of complement, resistant to 3,000 rads of radiation. In addition, the suppressor activity could be removed by treatment with carbonyl. Thus, this activity best can be associated with a macrophage. Comparison of suppressor macrophages from tumor-bearing animals with those obtained from normal animals (natural suppressors) indicates that on a per cell basis, macrophages from the tumor-bearing animals are more active (approximately threefold). There is no increase in the percentage of macrophages in the spleens of tumor-bearing animals (3.5 plus or minus 0.2%) compared to normal animals (3.3 plus or minus 0.1%) as evidenced by nonspecific esterase staining. These results indicate that suppressor macrophages from tumor-bearing animals differ qualitatively from suppressor macrophages obtained from normal animals. Studies were initiated to investigate tumor immunity in the tumor-bearing animals. We were unable to detect cytotoxic T\lymphocyte (CTL) or to generate these cells in a cell-mediated lympholysis assay system (CML). These findings suggest that either tumor immunity is not present (privileged site) or that the techniques employed were not adequate.